Polymer blends of ethylene-propylene-diene (EPDM) polymers with poly-.alpha.-monoolefin polymers, particularly with polyethylene, are known to the art (see U.S. Pat. Nos. 3,176,052; 3,328,486; 3,361,850; and 3,751,521; and Canadian Pat. No. 798,416). Such blends are useful to prepare a broad range of molded products such as tubing, toys, and household and automotive items. For many applications, the item must have high structural integrity and good tensile strength. Unfortunately, polymer blends of EPDM polymers with polyethylene often exhibit low tensile strength.
To achieve higher tensile strengths, curing or crosslinking agents have been added to such polymer blends to effect chemical changes in the blend (see U.S. Pat. Nos. 3,256,366; 3,564,080; 3,758,643; and 3,806,558). For example, polymer blends described in U.S. Pat. Nos. 3,785,643 and 3,806,558, which are stated to be thermoplastic in nature, are prepared by partially crosslinking the polymers, particularly the EPDM polymers. The polymer blends of the present invention, i.e., physical blends of EPDM polymers having a high ethylene length index (ELI) and polyethylene (PE) polymers, are thermoplastic in nature, yet do not use curing or crosslinking agents in their preparation. Additionally, the tensile strengths of the polymer blends of the invention are far superior to that predicted from the additive individual effects of the polymer components.
Reference is made to commonly assigned U.S. Pat. No. 3,919,358 which discloses thermoplastic polymer blends of (1) an EPDM polymer having a high degree of unstretched crystallinity with (2) polyethylene. The unstretched crystalline content of the EPDM polymer employed therein was between about 10 percent and about 20 percent as determined by an X-ray diffraction technique. Unexpectedly, it has now been discovered that an EPDM polymer having little or no unstretched crystallinity, but having a high ethylene length index (ELI) can be advantageously used with polyethylene to prepare thermoplastic polymer blends that exhibit superior tensile strengths.